The present invention relates to a color image forming apparatus of the type operable with a plurality of replaceable developing units each storing a developer of different color and, more particularly, to a remaining developer sensing method for such a color image forming apparatus which maintains the amounts of developers remaining in the individual developing units equal to each other at all times.
An electrophotographic copier, facsimile machine, laser printer or similar image forming apparatus has a developing unit which is often loaded with a two-component developer, i.e. a mixture of toner and carrier. In this kind of developing unit, the toner contained in the developer is sequentially consumed as the development is repeated and, hence, the amount of developer remaining in the developing unit is reduced little by little. It is a common practice to sense the remaining amount of developer every time the development is repeated a predetermined number of times and, when the sensed amount is smaller than a certain reference value, to supply a fresh developer to the developing unit. The remaining amount of toner may be sensed in terms of a load acting on an agitator which is disposed in a toner hopper of the developing unit or in terms of a pressure acting on a piezoelectric sensor which is mounted on the inner wall of the toner hopper, as proposed in the past.
A color image forming apparatus operable with a plurality of replaceable developing units each accommodating a developer of different color is extensively used. With this type of apparatus, too, it is necessary to sense the amounts of developers remaining in the individual developing units. It has been customary with such a color image forming apparatus to sense the amounts of developers remaining in the individual developing units under identical sensing conditions and at the same timing, i.e., without changing them from one color to another. This, however, fails to allow an equal amount of toner to remain in the toner hoppers of all of the developing units, inasmuch as the characteristics, especially the fluidity, differs from a developer or toner of one color to a developer or toner of another color. Specifically, assume that the amount of developer containing a toner of particular color which is consumed at a higher rate than the other toners is sensed under identical conditions with the other developers. Then, even when the toner of the particular color becomes scarce in the developer, a toner near-end condition, toner end condition or similar toner shortage condition will not be detected immediately. Should the amount of supply of the developer, i.e., fresh toner be lowered, the density of an image developed by the toner would become poor while the proportion of the toner to the carrier would decrease. The carrier eventually increased in concentration relative to the toner is apt to deposit on various component parts such as a photoconductive drum which is an image carrier for forming a latent image thereon.